sinker562

Create your prim, say .5 meters thick x 30 meters x 30 meters. Right-click the prim and select Edit. Select 'Object' from the General/Object/Features/Texture/Content toolbar. Use the 'Hollow Shape' dropdown to select the shape of the hole you want to create. Click the 'Hollow' up/down arrows to create a hole in the platform. Once you have the hole, you can place other prims around to make the shape you want. The advantage is that you can use significantly fewer prims to create your shape. Good luck.

You haven't said anything pertinent to the original question.